Residual current detection is a key in earth-fault protection systems of power systems. There are two methods to detect the residual current. In the first method, core balance current transformer (CBCT) is used. The output of the transformer is is adequate to the residual current of the system below the CBCT in the direction of power flow. That part of the system, which is above the CBCT is not protected by use of that CBCT. This first method can be used in electrical network systems with low earth current, e.g. the compensated network. For use in the electrical network system with large earth current this method is expensive.
The second method of residual current detection uses the phase-current information from the instrument transformers. This information is then summed and the result is the residual current. This method can be used in electrical network systems with large earth current, e.g. resistor earthed network. It cannot be used in electrical network system with low earth current due to the sensitivity of current transformers.